1. Field of the Invention
The present invention relates to a liquid crystal composition composed of a liquid crystal compound containing silacyclohexane ring only, or a mixture of it with a conventional liquid crystal compound not containing silicon.
2. Description of the Related Art
A liquid crystal display element makes use of the optical anisotropy and dielectric anisotropy of a liquid crystal substance. Depending on the mode of display, there are TN type (twisted nematic type), STN type (super-twisted nematic type), SBE type (super-birefringent type), DS type (dynamic scattering type), guest-host type, DAP type (deformed array phase type), OMI type (optical mode interference type), and other type of devices. The most general display device possesses a twisted nematic structure depending on Shutt-Hellfritt effect.
Although the properties required of liquid crystal substances used in such liquid crystal displays may be slightly different depending on the display mechanism, wide liquid crystal temperature ranges and stability against moisture, air, light, heat or electric field are common requirements for all display methods. Moreover, the liquid crystal materials are preferred to have low viscosity, short address time, low threshold voltage and high contrast when used in a cell. At the beginning of the line present, there is no known single compound satisfying all of these requirements, and from several to more than ten liquid crystal compounds or latent liquid crystal compounds are mixed for use. Accordingly, it is also important that the constituent components are mutually miscible easily.
In particular, in view of recent developments in multimedia and wide spread use of portable appliances, energy saving and lower driving voltage are demanded in liquid crystal displays. Of the power used in a direct-viewing type liquid crystal display, the backlight consumes about a half, and as the means of curtailing power use dramatically, the reflection type liquid crystal display is being intensively pursued.
The action modes of the reflection type liquid crystal display include DS (dynamic scattering) mode, ECB (electrically controlled birefringence) mode, GH (guest-host) mode, and others, and in certain types it is required to use a liquid crystal material with a negative value of .DELTA..di-elect cons. (dielectric constant anisotropy).
Hitherto known independent liquid crystal compounds with negative .DELTA..di-elect cons. values include: ##STR2## where R.sup.1 and R.sup.2 are represented by an alkyl or alkoxy group with each group having 1 to 10 carbon atoms (Japanese Patent Provisional Publication No. 61-26898), ##STR3## where R.sup.1 and R.sup.2 are an alkyl or alkoxy group with each group having 1 to 12 carbon atoms; the sum of q and r equals 1 or 2; Z represents a single bond, CH.sub.2 CH.sub.2, OCH.sub.2, OCO, or CO.sub.2 ; X.sup.1 and X.sup.2 represent H, F, Cl, Br, or CN (Japanese Patent Provisional Publication No. 60-199840), ##STR4## where R.sup.1 and R.sup.2 are non-substituted or substituted alkyl groups with each group having 1 to 15 carbon atoms, Q is CO or CH.sub.2 (Japanese Patent Provisional Publication No. 2-503436), and other compounds having a partial structure with positions 2 and 3 of a 1,4-substituted phenylene group substituted by F, CN, etc., and other compounds having a partial structure with axial position of 1 or 4 of a 1,4-substituted cyclohexylene group substituted by F, CN such as ##STR5## (Japanese Patent Provisional Publication No. 60-226835).
One of demerits of liquid crystal display is the narrowness of the viewing field angle. To improve this, various novel modes have been proposed. One of them is the in-plane switching mode. In this mode, the viewing field angle is excellent because it is extended to .+-.60 degrees both vertically and horizontally, but its shortcoming is the slow response speed. This is because it was forced to use Nn type liquid crystals (.DELTA..di-elect cons.&lt;0) of high viscosity.